State of the art high-power field-effect transistors such as gallium nitride (GaN) heterostructure field-effect transistors (HFETs) feature record high powers and breakdown voltages. Although these features make them extremely promising for various applications in power electronics, certain material and device specifics significantly limit their performance characteristics.
For example, FIG. 1 shows an illustrative schematic structure of a GaN-based HFET according to the prior art. The GaN-based HFET is essentially a normally-on device. That is, the device channel is conducting in the absence of the applied gate voltage. For most power electronics applications, normally-on devices are not acceptable since a gate voltage source failure can result in extremely high currents flowing through the power transistors and other connected circuit elements and result in partial or total damage of the blocks and systems.
One approach to achieve a normally-off condition in a GaN-based HFET includes removing a portion of the area under the gate, e.g., via etching and/or the like. For example, FIG. 2 shows an illustrative schematic structure of a recessed gate GaN-based HFET according to the prior art. A circuit-based approach uses a combination of GaN-based HFETs with normally-off silicon (Si)-based devices forming cascade connections, or Baliga pairs. For example, FIG. 3 shows an illustrative comparison of an AlGaN/GaN-based HFET with a cascade circuit according to the prior art. Both types of the above approaches lead to significant performance degradation such as excessive leakage currents, lower breakdown voltage and poor reliability in normally-off HFETs of the type shown in FIG. 2, or due to significant parasitic parameters and additional series resistance of Si devices in cascade type circuits shown in FIG. 3.
One approach to improve electric field uniformity and increase the breakdown voltage is a device with underlying p-layers (e.g., reduced surface field (RESURF) devices), which is illustrated in FIG. 4. However, the RESURF design does not achieve normally-off operation nor does it allow for the bottom gate control of the device state.